Electronic structure of strontium titanate

The spectra of three sets of optical functions for SrTiO₃ crystals are determined in a broad energy range of fundamental absorption. The calculations are carried out using the experimental reflectivity spectrum in the range 1–35 eV and two theoretical permittivity spectra in the ranges 0–30 eV and 0–14 eV. The special features of these spectra have been determined. The theoretical spectra of the optical functions are compared with the spectra determined using the experimental reflectivity spectrum.     

Synchrotron reflectivity spectrum, characteristic electron losses, and electronic structure of TlCl crystals

Spectra of two complete sets of TlCl fundamental optical functions are obtained in the energy range 0–25 eV using experimental reflectivity and characteristic electron loss spectra. The ?₂ and-Im?^?1 spectra are decomposed into components, and their main features are determined. The bands, ?₂, and the localization of transitions in the Brillouin zone are calculated. The experimental and theoretical transitions are compared with theoretical data on the electronic structure of a TlCl crystal.     

Spectra of characteristic losses and the electronic structure of sodium nitrite

The complete sets of optical functions for a NaNO₂ crystal are calculated in the energy range of 4 to 24 eV using the integral Kramers-Kronig relations, experimental reflectivity spectra, and theoretical spectra of permittivity. The main distinguishing features and patterns of optical properties are determined as functions of the direction of the light polarization vector. An analysis of the participation of electrons in the formation of optical spectra is presented.     

Electronic and crystal structures of isomorphic ZnP2 and CdP2

This paper reports on the results of precision investigations into the crystal structure of zinc diphosphide and cadmium diphosphide single crystals. The complete sets of fundamental optical functions are calculated for both crystals in the range 2.2–5.3 eV. The dielectric function is resolved into components for the first time. Three basic parameters of the oscillators are determined, and the electronic structures of ZnP₂ and CdP₂ are compared.     

Electronic structure and optical properties of lead chloride

Spectra for the sets of optical functions for lead chloride are obtained for polarizations E ‖ a and E ‖ b at 10 K in the range of 4 to 30 eV. The calculations are carried out using synchrotron reflectivity spectra and a software package. The main features of the spectra of optical functions were established; they include the energies for volume and surface plasmons of two types. The bands and the spectra of permittivity and other optical functions for lead chloride crystals are calculated using the FP LAPW method. There was good agreement between the theoretical calculations and the data of the experimental spectra of R and other optical functions calculated using these R spectra.     

Synchrotron reflectivity spectra and optical properties of rare-earth oxides

The spectra of the complete set of optical functions for nine sesquioxides of rare-earth elements of the R₂O₃ group are obtained at 300 K in the energy range 0–50 eV. The calculations are carried out on the basis of synchrotron reflectivity spectra and computer programs using the Kramers-Kronig relations. The main features of the optical spectra and their dependence on the nature of rare-earth elements were determined.     

Optical spectra and energy band structure of AgAsS2 crystals

Ground and excited states of three exciton series are observed in the region of fundamental absorption edge of AgAsS₂ crystals. The contours of exciton reflection spectra are calculated and the main parameters of excitons and energy bands are determined in the center of Brillouin zone. The optical reflection spectra are investigated at 30 K in E∥c and E⊥c polarizations in AgAsS₂ crystals in the region of 2-6 eV. The optical functions are calculated from the reflection spectra and a scheme of electronic transitions responsible for peculiarities of reflection spectra deep into the absorption band is proposed.     

Optical and photoelectric properties of TlInS2 layered single crystals

Single crystals of the layered compound TlInS₂ were grown by direct synthesis of their constituents. The spectral and optical parameters have been determined using spectrophotometric measurements of transmittance and reflectance in the wavelength range 200–2500 nm. Absorption spectra of thin layers of TlInS₂ crystals are used to study the energy gap and the interband transitions of the compound in the energy region 2–2.4 eV. The dispersion curve of the refractive index shows an anomalous dispersion in the absorption region and a normal one in the transmitted region. The direct and indirect band gaps were determined to be 2.34 and 2.258 eV, respectively. Photoconductivity measurements at room temperature resolve the structure that can be identified with the optical transition.     

Optical and magneto-optical studies of a multiferroic GaFeO<Subscript>3</Subscript> with a high Curie temperature

Single crystals of a noncentrosymmetric orthorhombic pyroelectric ferrimagnet Ga_2?xFe_xO₃ with a Curie temperature within 260–345 K have been grown by the flux method. It has been found that the electrical properties of the single crystals varied over a broad range from 10⁵ to 10¹³ Ω cm depending on the presence of transitionmetal oxide impurities. The dispersion relations for all three principal dielectric functions of orthorhombic GaFeO₃ have been determined in the range 0.7–5.4 eV by spectroscopic ellipsometry. The spectra of the dielectric functions of the orthorhombic Ga_2?xFe_xO₃ crystals are compared with the spectra of the trigonal crystals. The Faraday effect and second-harmonic generation are studied, and the law of the transition to the paramagnetic state has been determined. The crystallographic and magnetic contributions to the second-harmonic generation are analyzed.     

Spectra of the complete set of CdBr2 optical functions in a broad energy range of 3 to 30 eV

The spectra of the complete set of CdBr₂ optical functions at 35 K in the energy range 3–30 eV are obtained using experimental reflectivity spectra and the Kramers-Kronig relations. The main features of the structures of the optical spectra, including the energies of volume and surface losses, are determined. The results of an analysis of the effective number of electrons participating in transitions are presented.     

Electronic and optical properties of layered chalcogenide FeIn2Se4

We investigated the spectral-dependent dielectric function and temperature-dependent bandgap energy of layered chalcogenide FeIn₂Se₄ crystals. The critical-point energy and Lorentzian broadening were analyzed by fitting second-derivative spectra using the standard critical point model. The temperature effect of the bandgap energy was analyzed based on an analytical model considering both thermal lattice expansion and electron–phonon interactions. We also extensively analyzed the temperature dependence of absorption tails and identified their possible origins. The dielectric functions and absorption coefficient in the photon-energy range of 0.75–4.75 eV were obtained. The results also showed that optical phonon modes associated with the electron–phonon interactions could be closely related to the average phonon energy.     

Electronic structure and simulation of the dielectric function of β-FeSi2 epitaxial films on Si(111)

The optical functions of iron disilicide (β-FeSi₂) thin epitaxial films are calculated from the reflectance spectra in the energy range 0.1–6.2 eV with the use of the Kramers-Kronig (KK) integral relations. A comparison of the results of calculations from the transmittance and reflectance spectra and the data obtained from the reflectance spectra in terms of the Kramers-Kronig relations indicates that the fundamental transition at an energy of 0.87±0.01 eV is a direct transition. An empirical model is proposed for the dielectric function of β-FeSi₂ epitaxial films. Within this model, the specific features in the electronic energy-band structure of the epitaxial films are described in an analytical form. It is shown that the maximum contributions to the dielectric function and the reflectance spectrum in the energy range 0.9–1.2 eV are made by the 2D M ₀-type second harmonic oscillator with an energy of 0.977 eV. This oscillator correlates with the second direct interband transition observed in the energy-band structure of β-FeSi₂.     

Low-temperature time-resolved vacuum UV spectroscopy of NH<Subscript>4</Subscript>H<Subscript>2</Subscript>PO<Subscript>4</Subscript> crystals

A complex investigation of the dynamics of electronic excitations in nonlinear optical crystals of ammonium dihydrophosphate NH₄H₂PO₄ was performed using low-temperature vacuum UV luminescence spectroscopy with time resolution upon selective photoexcitation by synchrotron radiation. Data on the photoluminescence decay kinetics, time-resolved photoluminescence spectra (2–6.2 eV), and time-resolved photoluminescence excitation spectra (4–24 eV) were obtained for the first time for NH₄H₂PO₄ crystals at 8 K. It is ascertained that the photoluminescence of NH₄H₂PO₄ crystals in the vicinity of 4.7 eV has intrinsic character due to the radiative annihilation of self-trapped excitons. Possible channels of generation and decay of relaxed and unrelaxed electronic excitations in NH₄H₂PO₄ crystals are discussed.     

Optical properties of low-dimensional organic conductors with differently oriented conducting layers: (EDT-TTF)3Hg2Br6 and (EDT-TTF)3Hg(SCN)3I0.5(PhCl)0.5

Spectral optical investigations of two low-dimensional organic molecular conductors with differently oriented conducting layers of ethylenedithiotetrathiafulvalene (EDT-TTF) molecules, namely, the (EDT-TTF)₃Hg₂Br₆ and (EDT-TTF)₃Hg(SCN)₃I_0.5(PhCl)_0.5 single crystals, have been carried out. The polarized reflectance spectra of the single crystals have been measured in the frequency range 700–6500 cm⁻¹ (0.087–0.810 eV) at temperatures from 300 to 15 K. The optical conductivity spectra have been obtained using the Kramers-Kronig relations, and their quantitative analysis has been performed in terms of a theoretical model that takes into account electron-electron correlations in the approximation of the Hubbard Hamiltonian for trimerized stacks, the vibronic coupling, and the influence of the counterion on the electronic states in the trimer. A satisfactory agreement between the theoretical and experimental spectra for both crystals made it possible to estimate the parameters of the electronic structure of the crystals in the conducting plane: the integral t of the electron transfer between the EDT-TTF molecules in the trimer, the energy U of the Coulomb repulsion between two electrons (holes) in one EDT-TTF molecule, the electron transfer damping constant γ _e , the energy shift Δ of the molecular orbital under the influence of the anions and vibronic coupling, the vibronic coupling constant g _n , and the binding energy E _p of the molecular polaron. It has been found that there are large differences in the anisotropies of the optical properties and the obtained Hubbard parameters of the electronic structure for the studied crystals.

     

Analysis of reflectivity spectrum and band structure of Bi12GeO20

The fundamental reflectivity spectrum in the energy range 1.5–8.5 eV is reported for Bi₁₂GeO₂₀ (BGO) single crystals. From a Kramers-Kronig analysis the optical constants of BGO are computed. An energy level scheme for interband optical transitions is proposed. A value of 0.7 eV for the spin-orbit splitting of the valence band is suggested. Some features of the band structure are discussed, and the correlations between the band structures of BGO and A₂^VB₃^VI-compounds are indicated. The experimental premises for possible simplifications of a theoretical calculation of the band structure are given.     

Transient optical absorption induced by an electron pulse in KPb<Subscript>2</Subscript>Cl<Subscript>5</Subscript> crystals

The efficiency of formation and time evolution of radiation-induced structural defects and pulsed luminescence in KPb₂Cl₅ crystals under the action of a single electron pulse (E = 250 keV, τ = 20 ns) have been investigated. The spectra (1.1–3.8 eV) and relaxation kinetics (time interval 5 × 10^?8?5 s) of transient optical absorption and the pulsed cathodoluminescence spectra and decay kinetics (1.4–3.1 eV) have been measured in the temperature range 80–300 K. It is revealed that the induced optical density and its time evolution depend strongly on temperature, and the absorption relaxation time contains several components and reaches several seconds at T = 300 K. The decay kinetics of transient absorption and pulsed cathodoluminescence kinetics have different orders and are controlled by different relaxation processes.     

Optical properties of CdF<Subscript>2</Subscript> in a wide energy range

For either of the two reflection spectra of cadmium difluoride that are known from experiments, a complete set of the fundamental optical functions is calculated in the energy range 4–45 eV with the Kramers-Kronig relationships. The basic features of the optical spectra are established, and a hypothesis for their origin is suggested based on the known theoretical results for the band structure.     

Optical reflection spectra of cadmium halides

The reflection spectra have been measured at liquid helium temperature in the region 4–11.5 eV for CdCl₂ and CdBr₂ crystals. Exciton bands observed around the optical gap are associated with the halogen ion excitation under the influence of the strong crystal field. Characteristic sharp lines are also observed in the deep interband energy regions.     

Optical energy gaps of undoped and Co-doped ZnIn2Se4 single crystals

Single crystals of undoped and Co-doped ZnIn₂Se₄ were grown by the vertical Bridgman technique. The optical energy gaps of the single crystals were investigated in the temperature range of 10–300 K from the optical absorption measurements. The indirect optical energy gaps of the single crystals were found to be 1.624 eV for undoped ZnIn₂Se₄ and 1.277 eV for Co-doped one at 300 K. Also, the direct optical energy gaps were given by 1.774 and 1.413 eV for undoped ZnIn₂Se₄ and co-doped one, respectively. The temperature dependence of the optical energy gaps was well fitted by the Varshni equation.     

Electronic structure and optical properties of III–VI crystals

The polarized spectra of the full set of optical functions of GaS, GaSe, InSe, GaTe, InS, and TlSe crystals are determined in a wide range of fundamental-absorption energies. The ?₂ and -Im?^?1 spectra are decomposed into elementary components. The main parameters of the components were determined and the main features of the spectra and transition components are established. The results obtained are explained on the basis of the theoretical band calculations.